A conventional first stage circuit for an optical receiver comprises a differential amplifier including first and second transistors, first and second direct current bias circuits for applying a bias voltage to respective bases of the first and second transistors of the differential amplifier, and a photoelectric converting circuit including a photodiode for converting an output light emitted from an optical fiber to an electric signal. In the differential amplifier, collectors of the two transistors are connected through resistances to a power supply and directly to output terminals, and emitters thereof are connected through a common resistance to the ground. The photoelectric converting circuit is connected to the base of the first transistor.
In operation, when no light is emitted from the optical fiber, no electric signal is applied from the photoelectric converting circuit to the base of the first transistor of the differential amplifier except for the bias voltage, so that no amplified signal is obtained across the output terminals. On the other hand, when an output light is emitted from the optical fiber, a voltage which is proportional to the output light is applied from the photoelectric converting circuit to the base of the first transistor of the differential amplifier, so that the voltage is amplified in the differential amplifier to be provided at the output terminals.
However, the conventional first stage circuit for the optical receiver has a disadvantage in that a signal to noise ratio is deteriorated. In more detail, where noise is superposed, for instance, on a power supply terminal, an electric signal supplied from the photoelectric converting circuit to the base of the first transistor is affected by the noise, because the photodiode is connected through the terminal to the power supply. As a result, the noise is amplified in the differential amplifier together with the output signal of the photodiode, because the photoelectric converting circuit is connected only to the first transistor. On the contrary, noises supplied through the first and second bias circuits to the differential amplifier do not affect a signal to noise ratio, because the first and second bias circuits are connected to the bases of the first and second transistors symmetrically, so that noises are cancelled in the first and second transistors.
Although this kind of noise is reduced by a noise filter which is provided on a power supply line, as described on pages 209 to 225 of "NIKKEI ELECTRONICS, May 25, 1981", the provision of such a noise filter enlarges the first stage circuit for the optical receiver, and a signal to noise ratio is not completely improved due to noise which is not removed by the noise filter.